This disclosure relates generally to the field of optics and, more specifically, to systems and methods for correcting high-power beams of electromagnetic energy.
High power lasers are being considered for a variety of industrial, commercial, and military applications, including materials processing, satellite imaging, target tracking and identification, and directed energy weapons (DEW). Laser DEW systems generally involve the use of a high energy laser (HEL) to irradiate and destroy a target. To achieve performance objectives, many of these applications require that the laser beam be accurately steered and optimally focused. Steering involves line-of-sight control while focusing involves wavefront error correction.
Atmospheric turbulence produces density variations in the air that cause optical pathlength differences across a given beam path. The result is an optical distortion (or aberration) that reduces the average intensity of a focused laser beam due to beam spreading and causes spatial and temporal fluctuations in the beam due to scintillation. For many high power laser applications, it is advantageous to correct for the turbulence-induced aberration by pre-distorting the laser beam with the phase conjugate of the pathlength-integrated phase distortion (optical pathlength difference).
Traditional laser beam control adaptive optic (AO) systems use one or more multi-actuator deformable mirrors (DMs) in the beam path to correct for the wavefront aberrations caused by atmospheric turbulence. The conventional deformable mirror is typically a large element with a thin face sheet and a number of piezoelectric actuators. The outer surface of the face sheet is typically coated to be reflective for wavelengths of interest and is configured in the adaptive optical system as a mirror surface. Actuators are located behind the face sheet and are electrically driven to push and pull on the surface thereof to effect the deformation required to correct wavefront errors in an outgoing beam.
Astronomical telescopes routinely use DMs for atmospheric correction. Deformable mirrors may provide low and high spatial order correction. Two deformable mirrors may be employed in the same beam path to correct for both large-amplitude, low-frequency (temporal) and small-amplitude, high-frequency errors, respectively (“woofer/tweeter” arrangement).
However, deformable mirrors are difficult and expensive to manufacture and require a high throughput processor, called a real-time reconstructor. The real-time reconstructor is needed to calculate the actuator commands required to properly shape the mirror facesheet for optimal wavefront correction.
Previous attempts at achieving the “woofer” function have employed a full five degrees of freedom motion system to achieve the precision required for the desired degree of wavefront correction. These include U.S. Patent Publication No. 2003/0206350 A1 entitled “Low-Order Aberration Correction Using Articulated Optical Element” to Byren et al., U.S. Pat. No. 5,229,889 to Kittell, U.S. Patent Publication No. US 2003/0011073 A1 to Shinogi et al., and U.S. Pat. No. 6,278,100 B1 to Friedman et al. In particular, Friedman et al. discloses a rigid secondary mirror configured within an on-aperture (centrally obscured) Cassegrain telescope, the secondary mirror being articulated in multiple degrees of freedom with at least two actuators. The configuration of the actuators, as shown in the drawing, is such that the mirror will not translate in a lateral (non-focus) direction without also rotating.
Hence, a need exists in the art for an improved system or method for effecting aberration correction of a high power laser beam which is less expensive and less complex than conventional approaches.